The invention relates to the field of digital optical recording on disk-shaped information carriers. More specifically, the invention most closely relates to recording on rewriteable phase-change disks.
The invention relates to a method of recording marks representing data, at a predetermined writing speed or at different writing speeds in an information layer of an optical information carrier by irradiating the information layer with a pulsed radiation beam. Each mark is written by a sequence of one or more pulses having a first radiation power level. The invention also relates to an optical recording device for carrying out the recording method. The method is suitable for direct-overwrite on an information carrier, i.e. by writing information to be recorded in the information layer of the carrier and at the same time erasing information previously written in the information layer. The method can be used for direct-overwriting in information layers made of a phase-change material.
The writing speed is the magnitude of the velocity between the information layer of the information carrier and a spot formed by the radiation beam on this layer. When writing data on an information carrier, the writing speed may change as a function of the position of the irradiating beam on the information layer. Changes in writing speed are encountered when writing on a disc-shaped information carrier rotating at a constant angular velocity. This applies both when writing at a radius-independent data rate and at a radially increasing data rate.
A recording method according to the preamble is known from the Japanese patent application no. JP-A 3-283021, the above citations are hereby incorporated herein in whole by reference.
The known method is suitable for writing marks in an information layer at different speeds. The inventors have determined that this known method is not suitable for direct-overwrite.
It is an object of the invention to provide a recording method which provides a reliable direct-overwrite recording at different writing speeds.
In the method of the invention the last pulse in the sequence has a first power level and is followed by a second power level lower then the first power level during a cooling period and, subsequently, by a third power level higher than the second power level, the duration of the cooling period being dependent on the writing speed. The writing of marks is made by the pulses at the first power level. Erasure of previously written marks in the spaces between presently written marks is made by irradiation at the third power level. The cooling period following the last pulse of the sequence and preceding the start of the erasure provides cooling of the information layer at the end of the sequence. If the cooling period is too short, the erasure starts too soon and will erase too much of the just written mark. If, on the other hand, the cooling period is too long, the erasure starts too late and previously written marks immediately following the just written mark will not be erased. There is an optimum duration of the cooling period when writing at a certain speed. When changing the writing speed, it turns out that the duration of the cooling period must be changed depending on the writing speed in order to obtain a proper transition from the write operation to the erasure operation,
It is remarked that U.S. Pat. No. 5,109,373 to Ohno, hereby incorporated herein in whole by reference, discloses a pulse sequence for writing a mark in an information layer, the last pulse of the sequence being followed by a low power level during a certain period and, subsequently, a higher power level for erasure. However, the patent does not describe how the sequence must be modified when the writing speed is changed.
In a preferred embodiment of the method according to the invention the duration of the cooling period is linearly related to the inverse of the writing speed. Preferably, the length on the information layer corresponding to the cooling period has a constant value, independent of the writing speed and the type of information carrier.
Small variations in the number of pulses per unit of length of a mark are possible in embodiments where a disc-shaped carrier rotating at constant angular velocity is divided in several zones. Each of the zones is written at a constant angular density and zones at increasing radii are written at increasing angular density.
The length on the information layer corresponding to the cooling period depends on the overlap of the areas heated by S the last radiation pulse of the sequence and the start of the subsequent erasure. The size of a heated area is proportional to the size of the diffraction-limited spot formed by the radiation beam on the information layer. The length of the cooling period is therefore preferably proportional to 8/NA and lies in a range from 0.09 to 0.27 times 8/NA, where 8 is the wavelength of the radiation and NA is the numerical aperture of the radiation beam. This means that the cooling period has a duration preferably between 0.09 and 0.27 times 8/(NA*v), where v is the writing speed. In terms of channel-bit periods, the duration of the cooling period lies preferably within the range from 2.85*106 to 8.54*106 times 8(NA*v).
The pulses in a sequence for writing a mark have preferably a substantially equal pulse width, and a mark is written by a substantially constant number of pulses per unit of length of the mark independent of the writing speed. The control unit of a recording device can be simple, because the number of write pulses for forming a mark of a certain length need not be changed when the writing speed is changed. The combination of a substantial constant pulse width and an equal number of pulses per unit of length of the mark, provides an equal amount of radiation energy deposited per unit of length, resulting in the formation of marks which have a width independent of the writing speed. The method is very suitable for writing marks which can only have a discrete number of lengths, for instance, a length equal to an integer times a so-called channel-bit length. The number of write pulses for such a mark will then preferably be equal to the number of channel-bit lengths minus one or two.
The constant number of write pulses per unit of length and the equal width of the pulses does not apply to the leading and trailing edge of a mark. These edges, comprising together approximately one to two channel-bit lengths, form transient phenomena which are dealt with in special embodiments of the method according to the invention.
The pulses are preferably synchronized to a data clock signal, the frequency of which depends on the writing speed. When the frequency of the data clock is proportional to the writing speed, a substantially constant linear information density on the information layer can be realized. A coupling of the timing of the pulses to the data clock enables the proper formation of marks at all writing speeds. The coupling can be realized in the control unit by simple electronic means.
A simplification of the control unit of a recording device using the method, can be achieved by maintaining the power in the pulses substantially at a predetermined write level, independent of the writing speed and the length of the marks.
At relatively large speed differences, the write power preferably increases with decreasing write speed. The write power preferably has a linear dependence on the write speed. The advantage of this dependence is already noticeable when speed changes by a factor of 1.5. A decrease of the writing speed by a factor of two and a half and an associated increase of the write power in the range from 5% to 25%, improves the write performance. The higher power compensates for the increased cooling at low speeds due to the low duty cycle. A second aspect of the invention relates to a method of recording marks representing data at a writing speed in an information layer of an optical information carrier, by irradiating the information layer by a pulsed radiation beam. Each mark is written by a sequence of one or more pulses. In the method according to the invention the last pulse in the sequence has a first power level and is followed by a second power level, lower than the first power level, during a cooling period and, subsequently, by a third power level, higher than the second power level. The duration of the cooling period is between 0.009 and 0.27 times 8/(NA v), where 8 is the wavelength of the radiation of the beam, NA is the numerical aperture of the beam incident on the information layer and v is the wiring speed.
The length on the information layer corresponding to the cooling period depends on the overlap of the areas heated by the last radiation pulse of the sequence and the start of the subsequent erasure. The size of a heated area is proportional to the size of the diffraction-limited spot formed by the radiation beam on the information layer. When the duration of the cooling period is chosen in the indicated range, the corresponding length provides such an overlap of the heated areas that the rear edge of the written mark is defined properly. As a result, the jitter on reading the marks is reduced.
A third aspect of the invention relates to an optical recording device adapted for using the recording method according to the first aspect of the invention. The recording device records data in the form of marks on an information layer of an information carrier by irradiating the information layer by a radiation beam. The device includes a radiation source providing the radiation beam and a control unit for controlling the power of the radiation beam according to a writing speed. The control unit is operative for providing a sequence of one or more pulses having a first power level for writing a mark, the last pulse in the sequence being followed by a second power level during a cooling period and, subsequently, by a third power level. The duration of the cooling period is dependent on the writing speed.
A fourth aspect of the invention relates to an optical recording device adapted for using the recording method according to the third aspect of the invention. The optical recording device records data in the form of marks on an information layer of an information carrier at a writing speed v by irradiating the information layer by a radiation beam having a numerical aperture NA and a wavelength 8. The device includes a radiation source providing the radiation beam, and a control unit for controlling the power of the radiation beam according to a writing speed. The control unit is operative for providing a sequence of one or more pulses having a first power level for writing a mark, the last pulse in the sequence being followed by a second power level during a cooling period and, subsequently, by a third power level. The duration of the cooling period is between 0.09 and 0.27 times 8/(NA*v).
Those skilled in the art will understand the invention and additional objects and advantages of the invention by studying the description of the preferred embodiments below with reference to the following drawings which illustrate the features of the appended claims.